drain the bed. Remove the sand from within the tube.

Collect several such samples from well- scattered locations on the filter bed, mixing until about 2 pounds remain. Dry this sample; mix, quarter, and reduce to a usable sample size.

Determine loss of weight of a 10-gram sample during acid treatment. Treat the sample with 10 percent hypochloric acid in a Pyrex evaporating dish in a water bath for 24 hours. Replace acid loss during treatment period. Wash, dry, and weigh the sand. Determine the weight loss and compare it to the previous analysis.

From the remainder of the sand sample, remove 100 grams and run a sieve test. Compare the results to previous tests.

When either inspection, weight loss or sieve analysis shows growth of sand grains to a point where filtration efficiency is impaired, treat the sand as follows:

Add inhibited muriatic acid at the surface and allow it to pass downward through the bed and out the filter drain, or "rewash" the line; or add it to an empty filter through a small tap on the bed side of the wash waterline.

Adjust the water treatment process as necessary. If treatment is not effective, replace the filter medium. 8-12

Gravel Inspections

Gravel inspection procedures include the following:

1. At monthly intervals, check the gravel bed surface for unevenness, using a garden rake or a pole as a probe during backwashing. When ridges or sinkholes are shown, the filter may need overhauling.

2. Every 6 months, remove sand from an area about 3 square feet, taking care not to distribute the gravel. Examine the gravel by hand to determine if the gravel is cemented with incrustation or mud balls, or if it is not layered properly.

3. When any undesirable conditions exist to a marked degree, the sand should be removed and the filter gravel relaid. When unevenness or layer mixing is caused by a faulty underdrain system, repair it; when it is caused by faulty backwashing, correct the backwashing procedure.

Filter Underdrain Systems

Annually, or as observations indicate the need, the filter bottom should be inspected. Sand boiling during backwashing or sand craters on the surface indicate trouble in the underdrain system, as does marked unevenness of the gravel layers. Inspection and treatment procedures are as follows:

1. To inspect the bottom, remove the sand over an area of about 10 feet square. Select an area where sand boils or other indications of trouble have been noticed. Place planking over the gravel to stand on, and remove the gravel from areas about 2 feet square. Check the underdrains for deterioration. When the underdrains need repair, remove all sand and gravel, make repairs to the underdrain, and replace the gravel and sand in proper layers.

2. Where the underdrains are porous plate and are clogged with alum floc penetration, flood the underdrain system with a 2 percent sodium hydroxide solution for 12 to 16 hours.

Wash Water Troughs

At quarterly intervals, the level and elevation of troughs should be checked. Water should be drawn below the trough lip, the wash water valve cracked, and any low points observed where water spills over the lip, before the lip is covered completely.

The troughs should be adjusted as necessary to produce an even flow throughout their lengths on both sides.

At 6 month intervals, metal troughs should be inspected for corrosion. When corrosion exists, the troughs should be allowed to dry, and then cleaned by wire brush and painted with a protective paint or coating.

Operating Tables

Operating controls for filter valves may be mounted on a console, panel, or table. The controls actuate the filter valves which may be powered either by hydraulic or pneumatic means. The controls may be connected to the valve mechanism either mechanically, electrically, hydraulically, or pneumatically.